The present invention relates to a method for controlling and/or regulating the air pressure in a compressed air supply device for a utility or commercial vehicle, with the steps: detection of at least one pressure value in the compressed air supply device and/or in vehicle components connected to the compressed air supply device, determination of a moisture value which represents the moisture content in an air filter unit belonging to the compressed air supply device, and opening of a shiftable clutch connecting a drive to a compressor, and also opening of a discharge valve of the compressed air supply device when the pressure value lies above a predetermined minimum value and the moisture value overshoots a predetermined threshold value.
The invention relates, furthermore, to a compressed air supply device and to a utility vehicle having a compressed air supply device.
Compressed air supply devices are of a central importance for utility vehicles. In particular, the brake system of a pneumatically braked utility vehicle requires compressed air, as do numerous further consumers, for example air suspensions or lift axle devices. The compressed air to be prepared and distributed by the compressed air supply device is delivered by a compressor which is driven, in general, by the internal combustion engine of the utility vehicle. In a commonly accepted concept in this regard, the compressor is coupled to the internal combustion engine via a pneumatically shiftable clutch, the compressed air for the pneumatic activation of the clutch being extracted, in turn, from the compressed air supply device. An example of a device of this type is given in DE 3923882 C2.
Owing to the shiftability of the clutch, the operation of the compressor can be interrupted, as required, for example during the regeneration phases of the filter unit. With the clutch being opened, a conveyance of air through the filter unit in the direction of the consumer is interrupted while a regeneration of the filter unit is taking place, wherein dry air is conducted out of the reservoirs through the filter unit in the opposite direction. The dry air absorbs the moisture and can then escape via a discharge valve. Irrespective of the need for regeneration, the discharge valve is also opened when the pressure prevailing in the compressed air supply device reaches a maximum value. Functionality as a pressure controller is thereby made available by the compressed air supply device. One disadvantage to be noted in this regard is that any action of the pressure controller entails compressed air losses via the discharge valve and other switching losses related to the electrical, electropneumatic and pneumatic valves.
The object on which the invention is based is to make available a method having an improved energy balance with regard to the pressure controller function.
The invention builds on the generic method in that the clutch is opened and the discharge valve remains closed when it is indicated that the clutch can be engaged, the pressure value reaches a predetermined switch-off pressure and the moisture value lies below the predetermined threshold value. On the basis of the shiftability of the clutch between the internal combustion engine and compressor, the clutch can assume the pressure controller function independently of the discharge valve. When the pressure reaches the switch-off pressure, the clutch is opened with minimal compressed air consumption, whereas the discharge valve remains closed in order to avoid considerably higher compressed air losses. The discharge valve has to be opened only when regeneration is actually necessary, as can be detected by the overshooting of a moisture threshold value. Particularly when the vehicle is in an operating mode with low air consumption, a high degree of energy saving can thereby be achieved, as compared with pressure control via the discharge valve, since, with a low air consumption, the filter unit has to be regenerated only rarely. The discharge valve can to that extent remain closed while pressure control is nevertheless taking place via the shiftable clutch. By contrast, in the case of a high compressed air consumption, frequent regeneration will have to be carried out, and therefore the switching frequency of the switch-off valve rises. Thus, whereas, in the case of low air consumption, the energy saving takes place, in particular, on account of the long opening durations of the clutch and pressure control solely via the clutch, in the case of high air consumption the energy saving is made available, in particular, by means of directed regeneration when the moisture threshold value is reached. The moisture content in the air filter unit can be measured or calculated.
In the case of calculation of the moisture value, there is provision for the moisture value to be calculated, taking into account the conveyed air volume and the efficiency of the air filter unit. The conveyed air volume can be calculated from the characteristic data of the compressor and from the rotational speed of the compressor. The moisture content is then obtained by summing or integrating the calculated moisture contributions over time.
There may likewise be provision for the decrease in the efficiency of the air filter unit with the running time of the air filter unit to be taken into account in the calculation of the moisture value. Without the running time behavior being taken into account, an efficiency which is actually too low is assumed in the case of a new air filter unit. With the running time behavior being taken into account, the interval between the regeneration phases, particularly in the case of a new filter unit, can be increased considerably, thus leading to an additional energy saving.
For comparable reasons, it is expedient that the temperature of the surroundings is taken into account in the calculation of the moisture value. Warm air, as a rule, stores more water than cold air. If a temperature-dependent correcting factor is defined and if the correcting factors determined on the basis of this definition are adopted in the summation or integration for determining the moisture content of the air filter unit, then the average time between the regeneration phases can be further prolonged, particularly in winter.
According to a preferred embodiment of the invention, there is provision for the clutch to remain closed and for the discharge valve to be opened when the shiftability of the clutch is not afforded on account of a defect, the pressure value reaches a predetermined switch-off pressure and the moisture value lies below the predetermined threshold value. Conventional pressure control via the discharge valve thus offers a substitute function for the primarily employed pressure control via the shiftable clutch.
There is expediency provision for the at least one pressure value and the moisture value to be detected by an electronic control. The pressure control function can thus take place by means of the electronic control on the basis of the determined values, preferably taking into account other parameters relating to the compressed air supply system or, in general, to the vehicle.
According to a particularly advantageous embodiment of the invention, there is provision for the electronic control to be integrated into a compressed air preparation plant. This can be implemented, in practice, in that the conventional electronic control of the compressed air preparation plant is extended to include the functionality of clutch activation.
It is also conceivable, however, that the electronic control communicates via an interface with a control integrated into a compressed air preparation plant. On this basis, the conventional electronic control of the compressed air preparation plant can remain largely unchanged, and the shift functionality for the clutch can be available externally.
Furthermore, there may be provision for the at least one pressure value to be detected by a pressure switch which, when the switch-off pressure is reached, generates an electrical signal for opening the clutch. Such a pressure switch may assume sole management by pressure control or act redundantly to pressure control by the electronic control. In this case, the electrical signal generated by the pressure switch may be supplied directly to an electrically switchable valve for the pneumatic activation of the clutch or to the electronic control, so that the latter makes available the electrical signal for the pilot control of the pneumatic valve.
The invention relates, furthermore, to a compressed air supply device for carrying out a method according to the invention and to a utility vehicle having a compressed air supply device according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.